1. Field of the Invention
The present disclosure relates to a device and a method for correcting the reset noise and/or the fixed pattern noise of an active pixel for an image sensor.
2. Description of the Related Art
A conventional active pixel essentially comprises a photosensitive element, such as a photodiode, connected to a node, called the photosensitive node having a voltage varying according to the charge accumulated by the photosensitive element. The pixel further comprises three transistors, a select transistor for selecting the pixel, a reset transistor for resetting the electric charge of the photosensitive element, and a read transistor for providing a signal representative of the voltage at the photosensitive node.
The operation of such a pixel mainly comprises the following steps:
a reset step during which the reset transistor is on. The voltage at the photosensitive node is then brought to a determined constant voltage;
a charge step during which the reset transistor and the select transistor are blocked. The photosensitive element accumulates charges, which tends to vary the voltage at the photosensitive node; and
a read step during which the reset transistor is off and the select transistor is on. The voltage at the photosensitive node is measured by a read circuit, not shown.
A limitation of active pixel image sensors is the presence of a fixed pattern noise, which adds to the constant voltage determined at the photosensitive noise in the reset step. The fixed pattern noise corresponds to a random offset, due, for example, to the pixel read transistor, which is constant for a given pixel but which varies for each pixel of the sensor.
A known method for suppressing the fixed pattern noise of a pixel consists of measuring a first signal representative of the voltage at the photosensitive node at the end of a read step and of measuring a second signal representative of the voltage at the photosensitive node at the end of the next reset step. The fixed pattern noise is then suppressed by calculating the difference between the two measured signals.
Another limitation of active pixel image sensors is the presence of a random reset noise in the electric signals generated by the sensor pixels. Indeed, at the end of each pixel reset step, when the reset transistor switches from the on state to the off state, the voltage at the read node is set to a determined constant voltage to which a reset noise of different amplitude adds at each reset. The reset noise is then found on the photosensitive node in the next read step. The reset noise is particularly disturbing since it is preponderating with respect to the other noises in the analog signal acquisition chain.
Various methods exist to limit or correct the reset noise.
A first method is discussed in the document entitled “Analysis and enhancement of low-light-level performance of photodiode-type CMOS active pixel imagers operated with sub-threshold reset” by Pain, Yang, Ortiz, Wrigley, Hancock, and Cunningham, IEEE Workshop on CCDs and AIS, Nagano (Japan), pp. 140–142, June 1999. The technique discussed in this document, known as the “hard then soft reset”, consists of using a reset transistor operating under its conduction threshold. However, this technique only enables reducing the reset noise by a factor √{square root over (2)}.
A second method is discussed in the document entitled “Low Noise Readout using Active Reset for CMOS APS” by Fowler, Godfrey, Balicki, and Canfield, Proceedings of SPIE, vol. 3965, pp. 126–135, 2000. The technique discussed in this document consists of resetting the photodiode by using an amplifier to negatively feedback the reset noise. This technique exhibits certain disadvantages, especially a high number of transistors per pixel (6 transistors), the need for a high supply voltage since there are several cascade-assembled transistors, the need for a non-noisy voltage ramp, and an implementation which is not easily compatible with a pixel array.
A third known method consists of successively memorizing signals representative of the voltage at the photosensitive node after the reset step and at the next read step. The difference between the two memorized signals enables suppressing the reset noise added at the end of the reset step. However, the implementation of such a method has a high cost since it requires, for each sensor pixel, the keeping in memory of the signal representative of the voltage at the photosensitive node at the end of the reset step during the entire next charge step.